You might be able to get a small metal bending brake to make the 90 degree angles a little easier. I was looking at one for shaping the back panel for my case that would have been good for making fairly straightforward bends like the ones you were making in the attempts you showed and it was about $30. (This guy. )

For light duty work if you're doing enough mods/full case designs involving aluminum or mild steel, it may be worth looking into. (For more complicated stuff you could look into something like this, but it's a bit spendy for occasional modding.)